It doesn’t. Overlapping regions are twice as loud as the single non-overlapped region (assuming both samples have the same volume). I requested the cross-fade functionality as item 1 here:
I don’t know how common this feature is on other samplers (I never found it in Shortcircuit either) but it seems rather important to me; otherwise you’re going to get very uneven volume on an instrument with overlapping sample zones.
For the user-interface, I suggest the REAPER approach. Dragging two zones so that they overlap could automatically create a cross-fade, which would be superimposed as an envelope outline on the zone rectangle. This could then be adjusted as necessary.